1. Field of the Invention
This invention relates to soil nutrient extraction methods and, more particularly, to hot water nutrient extraction systems.
2. Description of Related Art
Increasing demand for soil analysis, prompted by environmental and economic factors, has intensified the need for an inexpensive, fast, convenient, and precise extraction method. Wet chemistry extractions for soil analyses are often labor intensive, complicated or environmentally hazardous. The extractions are generally tedious, typically involve expensive equipment, require considerable laboratory space, and normally involve the use of numerous chemical reagents. These factors render contemporary soil analysis methods prohibitively expensive for all but the most serious agronomic and horticultural pursuits. Simplified soil analysis procedures, if commercially available, are unlikely to extract multiple nutrients in one step and generally require varied reagent use for extractions.
The search for a simple, one-step, nutrient extraction method utilizing hot water was probably initiated early in the twentieth century by Konig (1906). Anion exchange resin, in combination with water, has been used to measure the total desorption of phosphorus (Amer, et al., 1955). Van der Paauw (1969) proposed that phosphorus (P) extraction efficiency could be improved by using a soil-water ratio of 1:50. Korschens, et al. (1984) used the Soxhlet extraction technique, which utilizes water, to extract and measure carbon (C) and nitrogen (N) in soils. Suntheim and Matzel (1985) used a continuous water extraction technique to determine phosphorus conent in soils. Most recently, a hot water percolation method to extract both macro and micro nutrients was reported for acidic soils of Hungary by Fulkey and Czinkota (1993). Using this method, a mixture consisting of a 30 g soil sample and 10 g of sand was placed on the filter of a coffee percolator and water preheated to 102-105.degree. C. was percolated through the sample under a pressure of 120 to 150 kilopascals (equivalent to about 1.2 to 1.5 bar or 17.4 to 21.75 lbs./in.sup.2) until about 100 to 500 ml of extract were collected. Results obtained from this latter extraction process compared favorably with plant uptake experiments for acidic soils. Fulkey and Czinkota determined that the time required to obtain 100 ml of extract for percolation through pure soil samples averaged 41 minutes, while an average of 188 minutes was required to obtain 500 ml of extract. Because of such unreasonably long percolation times, they determined that it was necessary to dilute the soil samples with sand. They determined that too much sand resulted in such rapid percolation that the amount of nutrients released into the accumulated extract was insufficient for accurate quantitative analysis of the soil sample. Through trial and error, it was determined that a mixture of 25% sand and 75% soil sample, by weight, provided the best combination of reasonable percolation times (an average of 2.6 minutes for 100 ml of extract) and adequate release of nutrients into the extract.
What is needed is an improved method for hot water extraction of nutrients from soil samples which is inexpensive, rapidly performed, capable of providing excellent analytical results for a variety of nutrients, and which does not require the mixing of sand with a soil sample. Ideally, such an improved method would make use of readily available equipment which could be used, without modification, to practice the improved method, or which could be readily and inexpensively modified for such practice.